C6: Metallic Bonding
Metallic bonding and properties of metals
📋 Key Definitions
Metallic bonding: The strong electrostatic attraction between positive metal ions and the sea of delocalised electrons. Metals form a giant lattice of positive ions surrounded by delocalised electrons.
Delocalised electrons: Electrons that have left their atoms and are free to move throughout the metallic structure. They come from the outer shell of metal atoms.
Alloy: A mixture of a metal with one or more other elements (usually another metal or carbon). Alloys are harder than pure metals because the different-sized atoms disrupt the regular arrangement of ions.
Shape memory alloy: An alloy that can return to its original shape after being deformed when it is heated. Example: Nitinol (nickel-titanium alloy) used in dental braces and stents.
🔗 How Metallic Bonding Works
In a metal, the atoms lose their outer shell electrons to become positive ions. These electrons become delocalised - they are no longer attached to any particular atom and can move freely throughout the entire structure. This creates a "sea" of delocalised electrons surrounding the positive metal ions in a giant lattice.
Key principle: The strong electrostatic attraction between the positive metal ions and the negative delocalised electrons holds the structure together. This is metallic bonding. The more delocalised electrons per atom, the stronger the metallic bonding.
Example - Sodium
Sodium atoms each lose 1 electron to become Na⁺ ions. The delocalised electrons form a sea around the Na⁺ ions. The electrostatic attraction between Na⁺ and the delocalised electrons is the metallic bond.
Example - Magnesium
Magnesium atoms each lose 2 electrons to become Mg²⁺ ions. There are twice as many delocalised electrons per ion compared to sodium, so the metallic bonding is stronger. This explains why magnesium has a higher melting point than sodium.
Example - Aluminium
Aluminium atoms each lose 3 electrons to become Al³⁺ ions. With three delocalised electrons per ion and a high charge, the metallic bonding is very strong, giving aluminium a high melting point.
🔬 Properties of Metals
| Property | Explanation |
|---|---|
| Good conductors of electricity | Delocalised electrons can move throughout the structure and carry charge |
| Good conductors of thermal energy | Delocalised electrons transfer energy quickly through the structure |
| Malleable (can be hammered into shape) | Layers of positive ions can slide over each other while still being held together by the sea of delocalised electrons |
| Ductile (can be drawn into wires) | Same reason - layers can slide without breaking the metallic bonds |
| High melting and boiling points | Strong electrostatic attraction between positive ions and delocalised electrons requires a lot of energy to overcome |
| Shiny/lustrous | Delocalised electrons reflect light |
Worked Example - Explaining Metal Properties
Why can metals be bent and shaped but ionic compounds are brittle?
Metals: When a force is applied, the layers of positive metal ions can slide over each other. The delocalised electrons move with them, so the metallic bonds are maintained and the metal does not break.
Ionic compounds: When a force is applied and layers shift, like-charged ions are brought next to each other (positive next to positive, negative next to negative). These like charges repel, causing the structure to shatter.
⚙️ Alloys
An alloy is a mixture of a metal with other elements. The different-sized atoms of the added element disrupt the regular arrangement of the metal ions, making it harder for the layers to slide over each other. This makes alloys harder than pure metals.
Why alloys are harder: In a pure metal, all the ions are the same size so the layers can slide easily. In an alloy, the different-sized atoms disrupt the layers, preventing them from sliding, making the alloy harder and stronger.
Examples of Alloys
- Steel: Iron alloyed with carbon (and sometimes other metals). Much harder than pure iron. Used in construction, tools and vehicles.
- Bronze: Copper alloyed with tin. Harder than pure copper. Used for statues, medals and musical instruments.
- Brass: Copper alloyed with zinc. Harder than pure copper. Used for door handles, musical instruments and decorative items.
- Solder: Alloy of tin and lead (now lead-free alternatives). Lower melting point than either pure metal. Used to join electrical components.
| Pure Metal | Alloy | |
|---|---|---|
| Structure | Regular layers of same-sized ions | Mixture of different-sized atoms disrupts layers |
| Hardness | Softer (layers slide easily) | Harder (layers cannot slide easily) |
| Properties | Uniform properties | Properties can be tailored for specific uses |
| Melting point | Fixed | Lower than the pure metal (usually) |
🔄 Shape Memory Alloys
Shape memory alloys are special alloys that "remember" their original shape. If they are deformed, they return to their original shape when heated.
Example - Nitinol
Nitinol is an alloy of nickel and titanium. It can be bent out of shape but returns to its original shape when warmed to a certain temperature.
- Used in dental braces - the wire returns to its shape at body temperature, gently moving teeth
- Used in surgical stents - compressed for insertion, then expands at body temperature
- Used in glasses frames - if bent, they can return to their original shape
❓ Practice Questions
Q1: Foundation Describe the structure of a metal and explain what holds the structure together.
Q2: Foundation Explain why metals are good conductors of electricity and heat.
Q3: Foundation Explain why pure metals are malleable but ionic compounds are brittle.
Q4: Higher Explain why an alloy is harder than a pure metal. Use a diagram description in your answer.
Q5: Higher Magnesium has a higher melting point than sodium. Explain this difference using ideas about metallic bonding.
✅ Answers
- A metal consists of a giant lattice of positive metal ions surrounded by a "sea" of delocalised electrons (electrons that have left the outer shell of the metal atoms). The strong electrostatic attraction between the positive ions and the negative delocalised electrons holds the structure together - this is metallic bonding.
- Metals conduct electricity because the delocalised electrons are free to move throughout the structure and can carry charge (current). Metals conduct heat because the delocalised electrons can transfer kinetic energy quickly through the structure when they gain energy from heating.
- Metals are malleable because the layers of positive ions can slide over each other while still being held together by the delocalised electrons - the metallic bonds are not broken. Ionic compounds are brittle because when a force causes the layers to shift, like-charged ions are brought next to each other. The strong repulsion between like charges causes the structure to shatter.
- In a pure metal, all the atoms are the same size so they form a regular arrangement. When a force is applied, the layers of same-sized ions can slide over each other easily. In an alloy, atoms of different sizes are mixed into the structure. These different-sized atoms disrupt the regular arrangement of the metal ions, making it much harder for the layers to slide over each other. This makes the alloy harder than the pure metal.
- Magnesium atoms lose 2 electrons each to become Mg²⁺ ions, while sodium atoms lose only 1 electron each to become Na⁺ ions. This means there are twice as many delocalised electrons per ion in magnesium compared to sodium. The greater number of delocalised electrons and the higher charge on the Mg²⁺ ions mean there is stronger electrostatic attraction between the ions and electrons in magnesium. This stronger metallic bonding requires more energy to overcome, giving magnesium a higher melting point.
🎯 Exam Tips
- When describing metallic bonding, always mention: positive ions, delocalised electrons, electrostatic attraction, and the sea of electrons
- The phrase "sea of delocalised electrons" is a key phrase examiners look for
- For explaining malleability: "layers of ions can slide" + "delocalised electrons move with them" + "metallic bonds are maintained"
- For alloys, always mention "different-sized atoms disrupt the layers" - this is the key explanation
- Comparing melting points of metals: more delocalised electrons = stronger metallic bonding = higher melting point
- Common mistake: saying "metal atoms" are in the lattice - it is "positive metal ions" (the atoms have lost their outer electrons)
🔢 Maths Skills
Mathematical Skills
Minimal maths in this topic. You may need to compare melting point data for different metals and relate it to the number of delocalised electrons per atom. More delocalised electrons per ion = stronger metallic bonding = higher melting point.
Example: Na (1 delocalised e⁻) mp 98°C; Mg (2 delocalised e⁻) mp 650°C; Al (3 delocalised e⁻) mp 660°C. The trend supports the model.
⚠️ Common Misconceptions
Watch Out!
Alloys are always stronger than pure metals. Wrong: alloys are always stronger Correct: alloys are harder (layers cannot slide easily), but "stronger" depends on the property measured — they are more resistant to deformation
Metals conduct because of free electrons. Wrong: metals conduct because of free electrons Correct: metals conduct because of delocalised electrons — the term "delocalised" is more precise because these electrons belong to the whole structure, not just one atom
✍️ 6-Mark Question
Extended Answer
6 marks: Explain why metals conduct electricity and why alloys are harder than pure metals.
Metals conduct electricity because the delocalised electrons in the metallic structure are free to move throughout the lattice. When a voltage is applied, these mobile electrons can carry charge through the metal, creating an electric current. Alloys are harder than pure metals because they contain atoms of different sizes mixed into the regular lattice. In a pure metal, all the ions are the same size so the layers can slide over each other easily. In an alloy, the different-sized atoms disrupt the regular arrangement, making it much harder for the layers to slide over each other. This makes the alloy harder and more resistant to deformation.
Mark scheme: 1 mark for delocalised electrons free to move; 1 mark for carrying charge / current; 1 mark for pure metal layers slide easily (same-sized ions); 1 mark for alloy has different-sized atoms; 1 mark for these disrupt the layers; 1 mark for layers cannot slide easily = harder.
📊 AO3: Analyse & Evaluate
Analysis and Evaluation
An engineer needs to select an alloy for a bridge cable that must be very strong and resistant to bending. Data for three alloys:
| Alloy | Composition | Hardness (relative) | Corrosion resistance |
|---|---|---|---|
| Mild steel | Fe + 0.2% C | Medium | Low |
| Stainless steel | Fe + Cr + Ni | High | High |
| Brass | Cu + Zn | Medium | Medium |
Question: Which alloy is most suitable? Justify your choice using the data.
Answer: Stainless steel — it has the highest hardness (different-sized Cr and Ni atoms disrupt the iron lattice most) and the highest corrosion resistance, which is important for a bridge exposed to weather.
📝 Exam Questions by Topic
🎬 Video Resources
Ready to ace your GCSE Combined Science exams?
Get the best revision books and guides to boost your grades.